The dyeing of dyeable materials, especially synthetic materials such as polyester, has previously been conducted with a dyestuff dispersed in an aqueous bath. The dyeable material is placed in the bath for a long enough time period to allow dyestuff to be absorbed to provide the desired coloration.
Such a dyeing process poses several disadvantages and limitations. Since the temperature of the aqueous bath cannot exceed the boiling temperature of the water, the process cannot be conducted at elevated temperatures unless high pressure is used. Even then, temperatures of only 250.degree. F. to 270.degree. F. are reached. Consequently, relatively long dyeing cycles are needed.
Additionally, the aqueous bath is generally disposed of after each dyeing cycle because most of the dyestuff has been absorbed by the dyeable material. The disposal of the used dye bath presents obvious environmental problems, as well as economic losses due to discharge of the residual dyestuff and chemicals remaining in the bath.
Dyeing at elevated temperatures with a non-aqueous system overcomes many of these problems and provides several advantages. Elevated temperatures reduce the time needed to dye the dyeable material. Shorter dyeing cycles make the process more economical and efficient.
Various dyeing processes that use non-aqueous dye compositions have been proposed for the treatment of dyeable materials. One technique involves immersing the dyeable material in a bath comprising an organic dyestuff dissolved in a high boiling aromatic ester or a cycloaliphatic diester. Such dyeing processes provide several disadvantages that prevent their effective and efficient use. The dye composition does not remain stable over a period of time when used in an ambient atmosphere and significant degradation of the dye composition often occurs after only a few hours of use.
It was initially believed that the stability problem could be eliminated by varying the chemical properties or components of the dye composition. Thus, in an attempt to solve the problem, the diester component was replaced by a modified diester, i.e., an adduct of terephthalic acid. Although this dye composition increased stability, significant degradation still occurred and satisfactory dyeing was not obtained. These poor results were especially prevalent when the dye composition was applied by spraying which exposes the largest liquid surface area to the atmosphere.